1. Field of the Invention
The present invention relates to an electric power steering apparatus having a steering system capable of flexibly setting a relationship between a steering angle of a steering wheel and a wheel angle of a tire.
2. Description of the Related Art
Conventionally, as is known in the related art, a steer-by-wire (SBW) system is the system in which a steering shaft as a drive shaft coupled to a steering wheel is mechanically separated from a driven mechanism setting a proceeding direction of tires independently. This system allows wider range of control options compared with a conventional steering system in which the steering wheel and the tires are coupled through a mechanical linkage. As an example, Japanese Unexamined Patent Application Publication No. 2000-128002 discloses a steer-by-wire control device in which a steering reaction force is determined based on the steering drive condition and the steering driven position. In the art disclosed in it, the steering reaction force acting on the steering shaft is determined by mainly using only a steering angle as the steering drive condition being input information.
However, in the above art, the steering reaction force is determined by considering only the condition in which the steering wheel is steered, and there is no consideration with respect to an appropriate value of the steering reaction force in a region of “on-center feeling”, which is the basis of the drivability. Here, this “on-center feeling” is due to a torque which is exerted on the steering wheel in case of slightly turning a vehicle. In this on-center region, if the torque to assist the steering of the steering wheel is too large, the driver cannot feel the center of the steering wheel (that is, straightly driving condition). As a result, there happens the problem that drivability is decreased. On the other hand, in this region, if this assist torque is too small, the drivability of straightly driving is increased, but there happens another problem that the driver's burden to turn the vehicle is increased.
Further, on-center feeling relates to a ratio (angle gain) of the wheel angle of the tire against the steering angle of the steering wheel. When this angle gain is small, the directional change of the tire becomes insensitive against operation of the steering wheel. Thus, while it becomes easy for the driver to keep the vehicle in a direction of straight driving, there happens the above mentioned problem on the driver's burden, because to turn the vehicle requires the driver wider range steering of the steering wheel. On the other hand, when the angle gain is large, since the directional change of the tire is sensitive against the operation, this results in an inconvenience in reverse to that occurring when the angle gain is small.